In a sensor, voltage of an output signal is changed proportionally to variation of the voltage of a power source. Typically the output is referred to as ratiometric output. In the case of ratiometric output, an error of a reference voltage generated between the sensor and a measuring unit is automatically corrected by using the same power source voltage as the power source voltage of the sensor as the reference voltage in the measuring unit (that is, an analog/digital converter (A/D) converter). PTL 1 discloses an example of a current sensor which has such a ratiometric output. According to the current sensor described in PTL 1, it is possible to measure a current value with good precision regardless of the variation of the power source voltage in combination with the A/D converter which uses the same power source voltage.
In addition, the current sensor described in PTL 1 senses the current using a magnetic sensor that uses a Hall element. That is, the current sensor senses a value of a measured current by sensing a magnetic field generated by the measured current using the Hall element. Meanwhile, a flux gate-type magnetic sensor, which is a well-known technology, is a magnetic sensor that does not use the Hall element. In the flux gate-type magnetic sensor, alternating current flows in an excitation coil which is wound on a magnetic core, and the magnetic core is periodically saturated. Then, a measured external magnetic field (hereinafter referred to as a “measured magnetic field”) is applied with respect to the magnetic core. When the external magnetic field is applied, a time interval of the magnetic saturation changes. The external magnetic field is measured using the phenomenon in which the time interval is changed. In the flux gate-type magnetic sensor, there is a sensor in which the external magnetic field is measured by directly measuring the time interval at which the magnetic saturation occurs, and there is a sensor in which the external magnetic field is measured by a feedback current flowing such that the time interval is a predetermined value in the coil which is wound on the magnetic core. The latter sensor is referred to as a closed-loop magnetic sensor, performs feedback control such that the external magnetic field is canceled, and passes current through a predetermined coil according to the external magnetic field.